Manufactured
by the Landsverk Electrometer Company (ca. 1953 – 1960) and designed for
the analysis of alpha, beta, and gamma radiation from a wide variety of
samples, e.g., water, food, filters etc.

The L-75 unit shown here came as part of the L-75KA analysis kit which
means that this was probably produced after 1953. The L-75 itself was
first manufactured around 1950. The black bakelite chamber is open on the
bottom so that it can be used to analyze the alpha, beta, or gamma
activity of a variety of samples, e.g., soil, water, food, filters. The
L-75KA kit comes with aluminum dishes and planchets to hold the samples
and aluminum absorbers for distinguishing alpha and beta activities.

The
original L-75 came in three versions: L-75A (chamber open at bottom),
L-75B (thin window over bottom of the chamber) and L-75C (closed thick
walled chamber). Around 1960 or so, the L-75D was introduced. The latter
used an internal transistorized charger powered by batteries.

It
is actually a modified version of a Lauritsen electroscope wherein the
sensing element ( a quartz fiber) is connected to an electrode that
projects down into a 200 cc
cylindricalion chamber on the
bottom of the unit. The chamber is three inches in diameter and the entire
unit is eight inches high.

To
operate, the rate of drift of the fiber across the scale, as viewed
through the eyepiece, is determined with a stopwatch or timer. In general,
a background rate of drift would be determined and then subtracted from
the drift rate determined for the sample.

The
145 volts or so required to zero the fiber is generated by the friction
charger knob on top of the unit. Since the capacitance of the device is
2.7 picofarads and a drop of 35 volts produces a full scale (100 division)
deflection, a collected charge of approximately 95 picocoulombs is
required for a full scale discharge (Q = V C). With a 300 cc chamber
volume, this roughly equates to an exposure of 1 mR.